In typical direct-melt glass fiber forming operations, raw batch ingredients are melted and homogenized in a furnace and fed to a refractory-lined forehearth having a plurality of openings in its bottom wall or floor. Each opening in the bottom wall of the forehearth is fitted with an insulating refractory block or "flow block" that has an elongated slot or bore extending therethrough to permit flow of the molten glass through the flow block. A second insulating refractory block or "bushing block", also having an elongated bore therethrough, may be positioned beneath the flow block to receive the molten glass from the flow block. A bushing having a plurality of apertures or tips in its bottom wall is positioned below the bushing block to receive molten glass from the bore of the bushing block. Continuous fibers are formed from the molten glass by attenuating streams of the molten material through the apertures or tips in the bottom wall (also called a tip plate) of the bushing using a winder or other attenuation device.
The temperature and flow velocity of the supply of molten glass in the forehearth varies depending upon the position of the molten glass relative to the bottom wall of the forehearth. Molten glass that is adjacent to the bottom wall of the forehearth typically has a lower temperature and flow velocity then the molten glass which is spaced apart from the bottom wall. These temperature and flow velocity variations are largely due to the transfer of heat from the molten glass to the refractory blocks lining the bottom of the forehearth.
Since the cooler molten glass adjacent the bottom wall of the forehearth tends to flow in a laminar manner through the end regions of the slot and the hotter molten glass tends to flow through the central region of the slot and there is little mixing of the molten glass as it passes through these regions, the variations in temperature and flow velocity are maintained or even increased due to heat loss by contact with the walls of the slot in the flow block and bushing block. The temperature difference between the portion of molten glass flowing through the central region of the slot and the portion of molten glass flowing through end regions of the slot in the bushing block can be up to about 125.degree. C. Similarly, the difference between the flow velocity of the molten glass flowing through the central region versus the peripheral region of the slot can be greater than about 0.01 meters per second (m/sec).
When the molten glass finally reaches the tip plate of the bushing, the molten glass adjacent to the end regions of the tip plate (which are aligned with the end regions of the flow and bushing blocks), often has a significantly lower temperature than the molten glass adjacent to the central region of the tip plate. Since the attenuation characteristics of molten glass are closely related to the temperature and flow velocity of the molten glass, non-uniform glass temperatures and flow velocities at the tip plate can contribute to production problems such as fiber diameter inconsistencies, fiber breakage during attenuation ("break-out") and low process efficiencies.
Attempts have been made to improve the temperature uniformity of molten glass by decreasing the amount of heat loss of the molten glass as it passes through the bushing block or by adding additional heat to the molten glass. For example, U.S. Pat. No. 4,249,398 discloses a bushing block having a wall with an insulating space and a radiation shield surrounding the insulating space to improve thermal isolation of the glass flowing through the block (col. 3, lines 62-65); U.S. Pat. No. 4,544,392 discloses the use of a bushing block with an auxiliary heater device to add heat to the molten material flowing through the ends of the bushing block (col. 1, lines 67-78 and col. 2, lines 1-2); and U.S. Pat. No. 4,161,396 discloses a bushing block with a substantially circular cylindrical flow passage configured to minimize the contact area between the glass and the surface of the flow passage to reduce the area available for heat transfer and heat loss in the glass (col. 1, lines 40-54 can col. 4, lines 6-14).
U.S. Pat. No. 5,709,727 discloses a bushing assembly including a glass receiving block having at least one cooling element interposed between the receiving block and the forehearth to remove heat from and redirect the flow of the molten glass to provide a more uniform temperature distribution throughout the glass (col. 2, lines 1-15).
U.S. Pat. No. 4,264,348 discloses a bushing block having a slot which is divided along its length into a plurality of separate sections having longitudinally inclined flow planes which divide glass flowing through the block into two or more portions along the length of the block and redirect the glass to emerge from the bushing block into areas substantially opposite to those in which it entered the block (col. 1, lines 55-62).
None of the foregoing references teach a simple, effective means for improving both the uniformity of the temperature and flow velocity of molten glass to improve fiber diameter consistency, reduce fiber break-out and improve process efficiency during fiber attenuation.